The family of a teenager believed to have been killed and buried in secret by the IRA in the 1970s have said a sixth search for him has ended in failure.

Relatives of Columba McVeigh, from County Tyrone, who was 19 when he disappeared in 1975, said they were devastated that the latest searches to recover his body had yielded nothing.

A dig for his remains began on 17 September when trees were cleared at a bog in Co Monaghan. McVeigh's brother Oliver said nothing had been found there and the organisation set up to locate him and other IRA victims – the Independent Commission for the Location of Victims – needed more information.

"There are people out there who have the information and they haven't come forward and I can't understand why," he said. "They have nothing to gain by keeping their secret and nothing to lose by telling the commission what they know. They know the information is confidential. Why do they continue to torture a family like this?"

His sister Dympna said: "Our mother went to her grave unable to tend the grave of her son. If the people who know where Columba is buried could have seen what that did to my mother, if they could imagine their own mother in that position, they could not stay silent if they had any human feelings at all. Let us bring Columba home and end this torment."

Since 1999 the commission has been searching for the bodies of those who disappeared during the Troubles, with nine bodies recovered. Seven remain missing: McVeigh, SAS Captain Robert Nairac, Joe Lynskey, Seamus Wright, Kevin McKee, Brendan Megraw and Seamus Ruddy.

The most infamous case of those whose bodies were recovered was that of Jean McConville. Her children along with the late Belfast IRA commander and hunger striker Brendan Hughes alleged that the Sinn Féin president, Gerry Adams, gave the order for the widow to be disappeared in 1972. Adams has strenuously denied any role in the McConville killing and rejects charges that he was in the Provisional IRA, let alone a senior commander of the organisation in Belfast at the time.